1. Field of the Invention
The present invention relates to a noise reduction apparatus for a digital camera, and in particular to noise reduction in long time exposure operations such as night-view imaging and astrophotography imaging.
2. Description of the Related Art
When a long time exposure operation is carried out in a digital camera which has an imaging device such as a CCD, there is a problem that a dark output by a certain picture element of the imaging device, becomes bigger than that of other picture elements due to the influence of the dispersion of dark current in each picture element on the imaging device, so that the dark output by a certain picture element turns out as a bright point in a dark area on the image.
Even for an optical black output, which is obtained when a partly shaded CCD is used, the dark current component is not able to be cut and removed, so that an image having generally increased brightness and damaged image quality is obtained.
Japanese unexamined patent publication (KOKAI) No. 2000-209506 discloses a noise reduction apparatus that images normally (normal exposure operation), stores the time length of an exposure operation, immediately exposes again while shading the CCD (the dark exposure operation) for the same length of time as the time length of the normal exposure operation, and reduces noise components by taking away a dark output obtained by the dark exposure operation from a normal output obtained by the normal exposure operation, for every picture element. In other words canceling the dark output of each picture element in the normal exposure operation by using the dark output of each picture element in the dark exposure operation, is the usual method of reducing the fixed pattern noise occurring due to dispersion of dark current in each picture element.
However, the above-discussed conventional noise reduction method does not consider the rise in temperature of the CCD associated with the time length of the dark exposure operation which is carried out immediately after the normal exposure operation. Dark current occurs due to heat, and the temperature of a CCD goes up according to the time length of the exposure operation.
Accordingly, an error results between the dark current component Im, obtained from the normal exposure operation, and the dark current component Id, obtained from the dark exposure operation, for every picture element. That is to say, the temperature of the CCD further goes up in the dark exposure operation, because the CCD has been used continuously from the normal exposure operation, so that the dark current in the dark exposure operation is more than the dark current in the normal exposure operation, corresponding to the rise in temperature of the CCD (Im<Id). The images obtained by taking away the dark current component Id, obtained from the dark exposure operation, from the normal output obtained by the normal exposure operation, create a condition where the dark output is pulled too much, accordingly the color balance of the image collapses and the brightness decreases (under exposure).